Question

(a) Distinguish between photodissociation and photoionization. (b) Use the energy requirements of these two processes to explain why photodissociation of oxygen is more important than photoionization of oxygen at altitudes below about \(90 \mathrm{~km}\).

Short answer

(a) Photodissociation is a process in which a molecule absorbs a photon and breaks up into smaller fragments, while photoionization is a process where an atom or a molecule absorbs a photon and loses one or more electrons, forming an ion. (b) At altitudes below 90 km, the energy required for the photodissociation of O2 is lower than its photoionization energy, making it more likely for O2 molecules to undergo photodissociation rather than photoionization. Hence, photodissociation of oxygen is more important than photoionization at altitudes below 90 km.

Step-by-step solution

Define photodissociation

Photodissociation is a process in which a molecule absorbs a photon (light) and breaks up into smaller fragments, often atoms or smaller molecules. It is a type of photochemical reaction, where changes in a chemical substance occur due to the interaction with electromagnetic radiation.

Define photoionization

Photoionization is a process where an atom or a molecule absorbs a photon (light) and loses one or more electrons as a result. This leads to the formation of an ion. Like photodissociation, photoionization is also a photochemical reaction involving the interaction of electromagnetic radiation with chemical substances.

Energy requirements of photodissociation and photoionization

The energy requirement for photodissociation depends on the bond dissociation energy, which is the energy required to break a specific chemical bond in a molecule. On the other hand, the energy requirement for photoionization depends on the ionization potential, which is the energy needed to remove an electron from an atom or ion.

Photodissociation and photoionization of oxygen at altitudes below 90 km

The oxygen molecule, O2, is the most abundant gas in the Earth's atmosphere up to about 90 km altitude. The energy required for the photodissociation of O2 is lower than its photoionization energy. This means that, at altitudes below 90 km, it is more likely for O2 molecules to undergo photodissociation rather than photoionization, as they can absorb lower energy photons which are more abundant in this region of the atmosphere. Consequently, photodissociation of oxygen becomes more important than photoionization at these altitudes.

Key concepts

Understanding Photochemical Reactions

Photochemical reactions are fascinating chemical processes that occur when light interacts with molecules. Think of them as transformations that wouldn't normally happen in the dark. When a molecule absorbs a photon, which is a particle of light, it gains energy. This extra boost can push the molecule to do things it normally wouldn't, like breaking bonds or losing electrons.

For example, in the photodissociation of oxygen (O2), a photon provides just enough energy for O2 to split into two atoms of oxygen (O). Similarly, during photoionization, the absorbed photon gives an electron in a molecule or atom so much energy that it can escape, creating an ion. These two types of photochemical reactions play a key role in atmospheric chemistry and have significant effects on our environment.

Bond Dissociation Energy Explained

Ever wonder what keeps molecules together? It's the chemical bonds between the atoms, and each bond has a certain strength, often referred to as bond dissociation energy. Picture a bond like a spring holding together two atoms; the energy required to snap that spring is the bond dissociation energy. It's a specific amount for each type of bond and is usually measured in kilojoules per mole (kJ/mol).

In the photodissociation process, absorbed light must provide energy equal to or greater than this bond energy to break the molecular bond. Factors like the type of bond and the atoms involved determine the necessary energy, which explains why different molecules absorb different wavelengths of light for these reactions to occur.

Ionization Potential Simplified

The term ionization potential, also known as ionization energy, is a concept that might seem complex at first, but it's really just a measure of how much energy it takes to remove an electron from an atom or molecule in its gas phase. It’s like the amount of effort required to pop a balloon - the balloon won't pop until you've applied enough force. Similarly, electrons will stay put until they absorb enough energy to overcome the attractive force of their nucleus.

Ionization energies are also measured in kJ/mol and are critical in determining the reactivity of an element or compound. In photochemistry, the ionization potential helps us understand why certain atoms or molecules are more likely to lose electrons when exposed to light. It's worth noting that the first ionization energy is always the lowest because it takes more energy to remove each successive electron due to increasing nuclear attraction.